1. Field of the Invention
The present invention relates generally to integrated circuits having on-chip redundant circuits and more particularly relates to a circuit for testing whether an integrated circuit has been modified to use a redundant circuit.
2. Description of the Relevant Art
It is well-known to include redundant circuits on an integrated circuit (IC) to increase the yield of the manufacturing process. For example, in memory multiple redundant rows and columns are often included to replace rows and columns that may be inoperative due to manufacturing defects. In many ICs a fuse link connects the input pin to the internal circuitry of the IC.
The IC is tested for detects during the manufacturing process. If the defect can be corrected by substituting an onboard redundant circuit for an inoperative circuit then the fuse link is disconnected, for example, by laser zapping, to substitute a functional redundant circuit for the defective main circuit. The manufacturing process is then completed and the chip is packaged and sold to customers. There is no indication on the final packaged ICs indicating whether redundant circuits have been activated.
In many instances, in particular where the design and process technology for a particular IC are not in a mature stage, manufacturers and end users may need to differentiate prime virgin devices from repaired devices because of differences in reliability and performance degradation. Accordingly, a fuse signature feature is incorporated in many ICs.
One example of a typical fuse signature circuit is depicted in FIG. 1. Typically, an IC includes power and ground pins connected to a power supply and a number of I/O pins for receiving and transmitting signals during normal operation. The allowable voltage levels for signals received on these pins are specified by the manufacturer. Referring now to FIG. 1, the fuse signature circuit 10 serially connects an input pin 12 to a power pin 14. The circuit includes a fuse 16 and three diode connected NMOS transistors 18, 20, and 22 coupled in series.
The state of the fusible link is tested by applying a test signal, having a test voltage level greater than 3*V.sub.TN, where V.sub.TN is the threshold voltage of the NMOS transistors 18, 20, and 22, to the input pin 12 and tying the power pin 14 to ground. If the fuse link is disconnected then no current flows through the input pin 12 and if the fuse link is connected then current does flow through the input pin.
This type of fuse signature circuit creates a problem when the IC is tested for short circuits or leakage current. During this test, the input pin is connected to a voltage terminal and all other pins are connected to ground. Unfortunately, the fuse signature circuit of FIG. 1 will allow current to flow through input pin 1 regardless of whether short circuits or leakage current are present in the IC. Accordingly, the short circuit test is difficult to perform when the above-described fuse signature circuit is included on the IC.